Separation membrane

ABSTRACT

A separation membrane ( 10 ) of the present disclosure includes: a separation functional layer ( 30 ) composed of a polyamide containing, as a monomer unit, at least one selected from the group consisting of piperazine and a piperazine derivative; and a coating ( 40 ) covering the separation functional layer ( 30 ) and containing a polymer having a repeating unit represented by the following formula (1). In the formula (1), N +  is a nitrogen atom constituting a quaternary ammonium cation, and R 1  and R 2  are each independently a substituent containing a carbon atom bonded to the nitrogen atom.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present disclosure relates to separation membranes.

2. Description of Related Art

Separation membranes are widely used as reverse osmosis membranes (RO membranes) or nanofiltration membranes (NF membranes) in various applications such as production of ultrapure water, desalination of seawater, and treatment of waste water. Examples of such separation membranes include composite semipermeable membranes having a porous support and a separation functional layer provided on the porous support. The separation functional layer is made of an organic compound such as polyamide, polysulfone, and cellulose acetate. In the field of reverse osmosis membranes, a polyamide membrane obtained by polymerization of an amine and an acid halide is known to be suitable as the separation functional layer. The polyamide membrane is typically an aromatic polyamide membrane obtained by interfacial polymerization of an aromatic polyfunctional amine and an aromatic polyfunctional acid halide. The porous support can be formed of a substrate such as a non-woven fabric and a microporous layer provided on the substrate.

In the field of membrane separation, a separation membrane capable of selectively separating monovalent ions and divalent ions from each other may be required. For example, sufficient separation between monovalent ions and divalent ions allows easy collection of valuable ions. JP 2016-190213 A describes a composite semipermeable membrane that achieves a magnesium sulfate removal ratio of 60% or more.

SUMMARY OF THE INVENTION

The divalent ion selective-separation performance of the composite semipermeable membrane described in JP 2016-190213 A is not necessarily satisfactory. Thus, a separation membrane capable of efficient selective separation of divalent ions has been demanded.

The present disclosure provides a separation membrane including:

-   -   a separation functional layer composed of a polyamide         containing, as a monomer unit, at least one selected from the         group consisting of piperazine and a piperazine derivative; and     -   a coating covering the separation functional layer and         containing a polymer having a repeating unit represented by the         following formula (1).

In the formula (1), N⁺ is a nitrogen atom constituting a quaternary ammonium cation, and R¹ and R² are each independently a substituent containing a carbon atom bonded to the nitrogen atom.

The technique of the present disclosure makes it possible to provide a separation membrane having superior divalent ion selective-separation performance.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a cross-sectional view of a separation membrane according to an embodiment of the present disclosure.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, an embodiment of the present disclosure will be described with reference to the drawings. The present disclosure is not limited to the following embodiment.

As shown in FIG. 1, a separation membrane 10 includes a porous support membrane 20, a separation functional layer 30, and a coating 40. The porous support membrane 20, the separation functional layer 30, and the coating 40 are stacked in this order. The separation functional layer 30 and the coating 40 are supported by the porous support membrane 20. The separation functional layer 30 is disposed on the porous support membrane 20. The coating 40 is disposed on the separation functional layer 30. The coating 40 is in contact with the separation functional layer 30. The separation membrane 10 can be a composite semipermeable membrane.

The separation functional layer 30 is composed of a polyamide containing, as a monomer unit, at least one selected from the group consisting of piperazine and a piperazine derivative. Such a polyamide exhibits superior divalent ion selective-separation performance. The coating 40 contributes to improvement in the divalent ion selective-separation performance of the separation membrane 10.

In the present specification, the divalent ion selective-separation performance is a property evaluated by the divalent ion rejection ratio in combination with the difference between the monovalent ion rejection ratio and the divalent ion rejection ratio. When the divalent ion rejection ratio is high and the monovalent ion rejection ratio is low, the divalent ion selective-separation performance can be considered superior. When the difference between the monovalent ion rejection ratio and the divalent ion rejection ratio is small, the divalent ion selective-separation performance cannot be considered superior even if the divalent ion rejection ratio is high.

The separation membrane 10 can be produced by the following method.

First, the porous support membrane 20 is prepared as a support. The porous support membrane 20 is not particularly limited as long as it is a membrane on the surface of which a separation functional layer can be formed. The porous support membrane 20 used may be an ultrafiltration membrane having a non-woven fabric on which a microporous layer with an average pore diameter of 0.01 to 0.4 μm is formed. Examples of the material forming the microporous layer include polyarylethersulfones such as polysulfone and polyethersulfone, polyimide, and polyvinylidene fluoride. From the viewpoint of chemical stability, mechanical stability, and thermal stability, polysulfone or polyarylethersulfone can be used. A self-supporting porous support membrane having an average pore diameter as specified above and made of a thermosetting resin such as epoxy resin can also be used. The thickness of the porous support membrane 20 is not particularly limited. The thickness is, for example, in the range of 10 to 200 μm and may be in the range of 20 to 75 μm.

In the present specification, the “average pore diameter” refers to a value calculated by the following method. First, a surface or cross-section of the membrane or layer is observed with an electron microscope (e.g., a scanning electron microscope), and the diameters of a plurality of observed pores (e.g., 10 randomly selected pores) are actually measured. The average of the actually measured diameters of the pores is defined as the “average pore diameter”. The “diameter of a pore” refers to the longest diameter of the pore, and specifically refers to the diameter of the smallest of the circles that can enclose the pore.

Next, a first solution containing a material of the separation functional layer 30 is brought into contact with the porous support membrane 20. The first solution is typically an aqueous solution containing a polyfunctional amine as the material of the separation functional layer 30 (this solution will hereinafter be referred to as “aqueous amine solution”). The contact of the aqueous amine solution with the porous support membrane 20 results in the formation of an amine-containing layer on a surface of the porous support membrane 20. The aqueous amine solution may contain, in addition to water, a polar solvent other than water, such as an alcohol. A polar solvent other than water, such as an alcohol, may be used instead of water.

The polyfunctional amine may be at least one selected from the group consisting of piperazine and a piperazine derivative. The piperazine derivative is a compound obtained by substitution of at least one hydrogen atom bonded to a carbon atom or nitrogen atom of piperazine with a substituent. Examples of the substituent include an alkyl group, an amino group, and a hydroxy group. Examples of the piperazine derivative include 2,5-dimethylpiperazine, 2-methylpiperazine, 2,6-dimethylpiperazine, 2,3,5-trimethylpiperazine, 2,5-diethylpiperazine, 2,3,5-triethylpiperazine, 2-n-propylpiperazine, 2,5-di-n-butylpiperazine, and 4-aminomethylpiperazine.

One compound selected from piperazine and the above piperazine derivatives may be used alone as the polyfunctional amine, or a combination of two or more selected from piperazine and the above piperazine derivatives may be used as the polyfunctional amine.

In order to facilitate the formation of the amine-containing layer and improve the performance of the separation functional layer 30, a polymer such as polyvinyl alcohol, polyvinylpyrrolidone, and polyacrylic acid, or a polyhydric alcohol such as sorbitol and glycerin may be added to the aqueous amine solution.

The concentration of the amine component in the aqueous amine solution may be in the range of 0.1 to 15 wt % and may be in the range of 1 to 10 wt %. When the concentration of the amine component is properly adjusted, the occurrence of defects such as pinholes in the separation functional layer 30 can be reduced. Additionally, the separation functional layer 30 having high salt rejection performance can be formed. Further, the proper adjustment of the concentration of the amine component leads to proper adjustment of the thickness of the separation functional layer 30, thus resulting in the separation membrane 10 capable of achieving a sufficient permeation flux.

The method for bringing the aqueous amine solution into contact with the porous support membrane 20 is not particularly limited. A method in which the porous support membrane 20 is immersed in the aqueous amine solution, a method in which the aqueous amine solution is applied to the porous support membrane 20, or a method in which the porous support membrane 20 is sprayed with the aqueous amine solution, can be used as appropriate. The step of bringing the aqueous amine solution into contact with the porous support membrane 20 may be followed by the step of removing the excess of the aqueous amine solution from the porous support membrane 20. For example, the excess of the aqueous amine solution can be removed from the porous support membrane 20 by extending the amine-containing layer with a rubber roller. The removal of the excess of the aqueous amine solution can result in the formation of the separation functional layer 30 of appropriate thickness.

Next, a second solution is brought into contact with the amine-containing layer. The second solution is a solution containing another material of the separation functional layer 30. Specifically, the second solution is a solution containing a polyfunctional acid halide as the other material of the separation functional layer 30 (this solution will hereinafter be referred to as “acid halide solution”). The contact of the acid halide solution with the amine-containing layer allows a polymerization reaction of the amine and the acid halide to proceed at the interface between the amine-containing layer and a layer of the acid halide solution. Thus, the separation functional layer 30 is formed.

The polyfunctional acid halide is an acid halide having a plurality of reactive carbonyl groups. Examples of the polyfunctional acid halide include an aromatic polyfunctional acid halide, an aliphatic polyfunctional acid halide, and an alicyclic polyfunctional acid halide.

Examples of the aromatic polyfunctional acid halide include trimesic acid trichloride, terephthalic acid dichloride, isophthalic acid dichloride, biphenyldicarboxylic acid dichloride, naphthalenedicarboxylic acid dichloride, benzenetrisulfonic acid trichloride, benzenedisulfonic acid dichloride, and chlorosulfonyl benzenedicarboxylic acid dichloride.

Examples of the aliphatic polyfunctional acid halide include propanedicarboxylic acid dichloride, butanedicarboxylic acid dichloride, pentanedicarboxylic acid dichloride, prop anetricarboxylic acid trichloride, butanetricarboxylic acid trichloride, pentanetricarboxylic acid trichloride, glutaryl halide, and adipoyl halide.

Examples of the alicyclic polyfunctional acid halide include cydopropanetricarboxylic acid trichloride, cyclobutanetetracarboxylic acid tetrachloride, cyclopentanetricarboxylic acid trichloride, cyclopentanetetracarboxylic acid tetrachloride, cyclohexanetricarboxylic acid trichloride, tetrahydrofurantetracarboxylic acid tetrachloride, cyclopentanedicarboxylic acid dichloride, cyclobutanedicarboxylic acid dichloride, cyclohexanedicarboxylic acid dichloride, and tetrahydrofurandicarboxylic acid dichloride.

One polyfunctional acid halide selected from these polyfunctional acid halides may be used alone, or two or more selected from these polyfunctional acid halides may be used in combination. An aromatic polyfunctional acid halide may be used in order to obtain the separation functional layer 30 having high salt rejection performance. A polyfunctional acid halide having three or more valences may be used as at least part of the polyfunctional acid halide component to form a cross-linked structure.

As the solvent of the acid halide solution there can be used an organic solvent, in particular a non-polar organic solvent. The organic solvent is not particularly limited as long as the organic solvent has low solubility in water and can dissolve the polyfunctional acid halide component without deteriorating the porous support membrane 20. Examples of the organic solvent include saturated hydrocarbons such as cyclohexane, heptane, octane, and nonane and halogen-substituted hydrocarbons such as 1,1,2-trichlorotrifluoroethane. A saturated hydrocarbon having a boiling point of 300° C. or lower or 200° C. or lower may also be used.

The concentration of the acid halide component in the acid halide solution may be in the range of 0.01 to 5 wt % and may be in the range of 0.05 to 3 wt %. When the concentration of the acid halide component is properly adjusted, the amounts of the amine and halide components remaining unreacted can be reduced. Additionally, the occurrence of defects such as pinholes in the separation functional layer 30 can be reduced, and thus the separation membrane 10 having high salt rejection performance can be provided. Further, the proper adjustment of the concentration of the acid halide component leads to proper adjustment of the thickness of the separation functional layer 30, thus providing the separation membrane 10 capable of achieving a sufficient permeation flux.

The method for bringing the acid halide solution into contact with the amine-containing layer is not particularly limited. The amine-containing layer may be immersed in the acid halide solution together with the porous support membrane 20, or the acid halide solution may be applied to a surface of the amine-containing layer. The time of contact between the amine-containing layer and the acid halide solution is, for example, 10 seconds to 5 minutes or 30 seconds to 1 minute. The contact between the amine-containing layer and the acid halide solution may be followed by the step of removing the excess of the acid halide solution from the amine-containing layer.

Next, the separation functional layer 30 is heated and dried together with the porous support membrane 20. The heat treatment of the separation functional layer 30 can improve the properties such as the mechanical strength and the heat resistance of the separation functional layer 30. The heating temperature is, for example, 70 to 200° C. or 80 to 130° C. The heating time is, for example, 30 seconds to 10 minutes or 40 seconds to 7 minutes. A drying step may be carried out at a room temperature, and subsequently another drying step may be carried out using a dryer at an ambient temperature higher than the room temperature.

The conditions under which interfacial polymerization is carried out are described, for example, in JP 58-24303 A and JP 1-180208 A. Such known techniques can be employed in the method of the present embodiment.

Various additives can be added to the aqueous amine solution and/or the acid halide solution in order to facilitate the formation of the separation functional layer 30 or improve the performance of the separation membrane 10 to be obtained. Examples of the additives include: a surfactant such as sodium dodecylbenzenesulfonate, sodium dodecyl sulfate, and sodium lauryl sulfate; a basic compound, such as sodium hydroxide, trisodium phosphate, and triethylamine, which has an effect on removal of halogenated hydrogen produced as a result of polymerization; an acylation catalyst; and a compound as described in JP 8-224452 A which has a solubility parameter of 8 to 14 (cal/cm³)^(1/2).

By carrying out the above steps, a membrane having the porous support membrane 20 and the separation functional layer 30 is obtained. The thickness of the separation functional layer 30 is not particularly limited. The thickness is, for example, 0.05 to 2 μm and may be 0.1 to 1 μm.

The present specification describes a method in which the separation functional layer 30 is formed directly on a surface of the porous support membrane 20 by interfacial polymerization. It should be noted that the separation functional layer 30 may be formed on a support other than the porous support membrane 20, and the separation functional layer 30 obtained may be placed on and joined to the porous support membrane 20. In other words, the separation functional layer 30 may be transferred onto the porous support membrane 20 from the other support.

Next, a solution containing a material of the coating 40 is brought into contact with the separation functional layer 30. The material of the coating 40 can be a polymer having a repeating unit represented by the following formula (1).

In the formula (1), N⁺ is a nitrogen atom constituting a quaternary ammonium cation. R¹ and R² are each independently a substituent containing a carbon atom bonded to the nitrogen atom.

When the coating 40 contains the polymer having the repeating unit represented by the formula (1), the divalent ion selective-separation performance of the separation membrane 10 is improved. The quaternary ammonium cation is always positively charged irrespective of the pH of raw water. This prevents adhesion of cationic substances to the surface of the separation membrane 10.

In the formula (1), the counterion for N⁺ is not particularly limited. The counterion for N⁺ is a monovalent anion. Examples of the monovalent anion include halogen ions such as F⁻, Cl⁻, Br⁻, and I⁻.

In the formula (1), R¹ and R² may each be an alkyl group. Examples of the alkyl group include a methyl group, an ethyl group, and a propyl group. In particular, R¹ and R² may each be a methyl group. When R¹ and R² are each an alkyl group such as a methyl group, the coating 40 can sufficiently improve the divalent ion selective-separation performance of the separation membrane 10. When R¹ and R² are each an alkyl group such as a methyl group, the coating 40 causes less influence on the salt rejection ratio of the separation membrane 10.

In the formula (1), R¹ may be a methyl group, while R² may be a 3-chloro-2-hydroxypropyl group. In this case, the repeating unit of the polymer is represented by the following formula (2).

In the formula (1), R¹ may be a methyl group, while R² may be a 2,3-epoxypropyl group. In this case, the repeating unit of the polymer is represented by the following formula (3).

When an alkali is allowed to act on the repeating unit represented by the formula (2), the 3-chloro-2-hydroxypropyl group undergoes a cyclization reaction. This reaction converts the repeating unit represented by the formula (2) to the repeating unit represented by the formula (3).

The polymer contained in the coating 40 can be a copolymer of a first monomer and a second monomer. The first monomer can be a monomer containing a quaternary ammonium cation and serving to form the repeating unit represented by the formula (1). The first monomer can be 3-chloro-2-hydroxypropylmethyl diallyl ammonium chloride. When the second monomer is diallylmethylamine hydrochloride, the copolymer is represented by the following formula (4).

In the formula (4), m and n are each independently an integer of 1 or more. Part or all of the 3-chloro-2-hydroxypropyl groups contained in the polymer of the formula (4) may be a 2,3-epoxypropyl group as shown in the formula (3).

In the copolymer represented by the formula (4), the 3-chloro-2-hydroxypropyl group and/or the 2,3-epoxypropyl group contained in the first monomer can be a reactive substituent capable of being chemically bonded to the separation functional layer 30.

The polymer of the formula (4) can be obtained also by modification of a homopolymer. Specifically, a homopolymer of methyl diallylamine hydrochloride can be modified with epichlorohydrin to obtain the polymer of the formula (4).

The reactive substituent enhances the bond strength between the separation functional layer 30 and the coating 40. Specifically, at least part of the reactive substituents form a covalent bond with a terminal amino group, a remaining amino group, or a remaining carbonyl group of the separation functional layer 30. Thus, the coating 40 is secured to the separation functional layer 30, so that the separation membrane 10 resistant to deterioration of divalent ion selective-separation performance even in long-term use can be provided. The terminal amino group and the remaining amino group of the separation functional layer 30 are derived from the polyfunctional amine. The remaining carbonyl group of the separation functional layer 30 is derived from the polyfunctional acid halide. The reactive substituent may be used in intramolecular cross-linking and/or intermolecular cross-linking of the polymer. Such cross-linking can improve the properties such as the mechanical strength and the heat resistance of the coating 40.

The reactive substituent may be contained in the second monomer. When the first monomer has a quaternary ammonium cation structure, fewer restrictions are imposed on the second monomer. That is, there is a lot of flexibility in choosing the second monomer.

The reactive substituent is not limited to a 3-chloro-2-hydroxypropyl group. Examples of the reactive substituent include an epoxy group, a hydroxy group, an amino group, and an amide group. One substituent selected from these reactive substituents may be contained alone in the polymer, or two or more selected from these reactive substituents may be contained in the polymer.

When the reactive substituent is contained in the second monomer, the second monomer is, for example, allylamine. The polymer is represented by the following formula (5).

In the formula (5), m and n are each independently an integer of 1 or more.

When the reactive substituent is contained in the second monomer, the second monomer is, for example, acrylamide. The polymer is represented by the following formula (6).

In the formula (6), m and n are each independently an integer of 1 or more.

When the reactive substituent is contained in the second monomer, the second monomer is, for example, 3-chloro-2-hydroxypropyl diallyamine hydrochloride. The polymer is represented by the following formula (7).

In the formula (7), m and n are each independently an integer of 1 or more.

One monomer selected from 3-chloro-2-hydroxypropyl diallylamine hydrochloride, allylamine, and acrylamide may be used alone as the second monomer, or two or more selected from these monomers may be used as the second monomer.

The copolymer may be a random copolymer or a block copolymer.

The ratio between the first monomer and the second monomer is not particularly limited. For example, the ratio (first monomer:second monomer) is 5:95 to 95:5 and may be 30:70 to 70:30. When the ratio is within this range, the separation membrane 10 having superior divalent ion selective-separation performance and usable over a long period of time can be provided. The weight-average molecular weight of the polymer or copolymer is not particularly limited and is, for example, 10,000 to 100,000.

The coating 40 can be formed by bringing an aqueous solution containing the polymer into the separation functional layer 30 to form a polymer-containing layer and then drying the polymer-containing layer. The method for bringing the aqueous solution into contact with the separation functional layer 30 is not particularly limited. The separation functional layer 30 may be immersed in the aqueous solution together with the porous support membrane 20, or the aqueous solution may be applied to a surface of the separation functional layer 30. The time of contact between the separation functional layer 30 and the aqueous solution is, for example, 10 seconds to 5 minutes. The contact between the separation functional layer 30 and the aqueous solution may be followed by the step of removing the excess of the aqueous solution from the separation functional layer 30. The aqueous solution may contain, in addition to water, a polar solvent other than water, such as an alcohol. A polar solvent other than water, such as an alcohol, may be used instead of water.

Next, the polymer-containing layer is heated and dried. The heat treatment of the polymer-containing layer can improve the properties such as the mechanical strength and the heat resistance of the coating 40. The heating temperature is, for example, 80 to 150° C. The heating time is, for example, 30 to 300 seconds. A drying step may be carried out at a room temperature, and subsequently another drying step may be carried out using a dryer at an ambient temperature higher than the room temperature.

By carrying out the above steps, the separation membrane 10 having the porous support membrane 20, the separation functional layer 30, and the coating 40 is obtained. The thickness of the coating 40 is not particularly limited and is, for example, 10 to 900 nm. The presence of the coating 40 can be confirmed by means of a transmission electron microscope. The composition analysis of the polymer contained in the coating 40 can be carried out by Fourier-transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), or time-of-flight secondary ion mass spectrometry (TOF-SIMS).

EXAMPLES Example 1

An aqueous amine solution containing 7 wt % of piperazine, 0.15 wt % of sodium dodecyl sulfate, 1.48 wt % of sodium hydroxide, and 6 wt % of camphorsulfonic acid was applied to a porous polysulfone support. After that, the excess of the aqueous amine solution was removed from the support to form an amine-containing layer on the support. Next, the surface of the amine-containing layer was immersed for 10 seconds in an acid halide solution obtained by solving 0.42 wt % of trimesic acid trichloride in an isoparaffinic solvent (IP Solvent 1016, manufactured by Idemitsu Kosan Co., Ltd.). After that, the excess of the acid halide solution was removed from the amine-containing layer, and the amine-containing layer was air-dried for 60 seconds and then placed in a hot air dryer at 120° C. for 3 minutes to form a separation functional layer on the porous polysulfone support. Next, the surface of the separation functional layer was immersed for 10 seconds in an aqueous solution containing 0.1 wt % of a polymer (UNISENCE KCA 101L, manufactured by SENKA Corporation). After that, the separation functional layer was air-dried for 30 seconds and then placed in a hot air dryer at 120° C. for 2 minutes to form a coating on the separation functional layer. In this manner, a separation membrane of Example 1 was obtained. UNISENCE KCA 101L is a polymer represented by the formula (4).

Example 2

A separation membrane was obtained in the same manner as in Example 1, except that the concentration of the polymer in the aqueous solution was changed to 0.03 wt %.

Example 3

A separation membrane was obtained in the same manner as in Example 1, except that the concentration of the polymer in the aqueous solution was changed to 0.01 wt %.

Example 4

A separation membrane was obtained in the same manner as in Example 1, except that the polymer was changed to PAS-880 manufactured by Nittobo Medical Co., Ltd. and that the concentration of the polymer in the aqueous solution was changed to 0.05 wt %. PAS-880 is a copolymer represented by the formula (7). R¹ and R² are each a methyl group.

Example 5

A separation membrane was obtained in the same manner as in Example 1, except that the polymer was changed to PAS-880 manufactured by Nittobo Medical Co., Ltd.

Example 6

A separation membrane was obtained in the same manner as in Example 1, except that the polymer was changed to PAS-J-81 manufactured by Nittobo Medical Co., Ltd. PAS-J-81 is a copolymer represented by the formula (6). R¹ and R² are each a methyl group.

Example 7

A separation membrane was obtained in the same manner as in Example 1, except that the polymer was changed to PAA-1123 manufactured by Nittobo Medical Co., Ltd. PAA-1123 is a copolymer represented by the formula (5). R¹ and R² are each a methyl group.

Example 8

A separation membrane was obtained in the same manner as in Example 1, except that the polymer was changed to PAS-H-5L manufactured by Nittobo Medical Co., Ltd. and that the concentration of the polymer in the aqueous solution was changed to 0.05 wt %. PAS-H-5L is a polymer represented by the formula (1). R¹ and R² are each a methyl group. That is, PAS-H-5L is a homopolymer of diallyldimethylammonium chloride.

Example 9

A separation membrane was obtained in the same manner as in Example 1, except that the polymer was changed to a mixture of the polymers used in Examples 1 and 4 and that the respective concentrations of the polymers in the aqueous solution were 0.03 wt % and 0.02 wt %.

The polymers used in Examples 4 to 9 belong to diallyldimethylammonium chloride polymers.

Comparative Example 1

A separation membrane was obtained in the same manner as in Example 1, except that the polymer was changed to polyvinyl alcohol.

Comparative Example 2

A separation membrane was obtained in the same manner as in Example 1, except that the polymer was changed to polyquaternium-10 (CATINAL HC-100, manufactured by TOHO Chemical Industry Co., Ltd.). CATINAL HC-100 is cellulose containing quaternary ammonium cation.

Comparative Example 3

A separation membrane was obtained in the same manner as in Example 1, except that no coating was formed on the surface of the separation functional layer.

Performance Evaluation: Rejection Ratio

The MgSO₄ rejection ratio and the NaCl rejection ratio were measured for the obtained separation membranes. In the measurement of the MgSO₄ rejection ratio, an aqueous MgSO₄ solution having a temperature of 25° C., a pH of 6.5 to 7, and a concentration of 2000 mg/liter was used. In the measurement of the NaCl rejection ratio, an aqueous NaCl solution having a temperature of 25° C., a pH of 6.5 to 7, and a concentration of 2000 mg/liter was used.

The aqueous MgSO₄ solution or aqueous NaCl solution was passed through each of the separation membranes of Examples and Comparative Examples at an operating pressure of 1.5 MPa for 30 minutes. An electrical conductivity meter (CM-117, manufactured by Kyoto Electronics Manufacturing Co., Ltd.) was used to measure the electrical conductivity of the permeate liquid and the feed liquid. From the measurement result and a calibration curve (concentration versus electrical conductivity), the MgSO₄ rejection ratio and the NaCl rejection ratio were calculated according to the following equations. The results are shown in Table 1.

-   -   MgSO₄ rejection ratio (%)=(1−(MgSO₄ concentration of permeate         liquid/MgSO₄ concentration of feed liquid))×100     -   NaCl rejection ratio (%)=(1−(NaCl concentration of permeate         liquid/NaCl concentration of feed liquid))×100

TABLE 1 Concentration MgSO₄ NaCl of polymer rejection rejection Type of polymer (%) ratio (%) ratio (%) Example 1 UNISENCE KCA 0.10 99.8 59.7 101L Example 2 UNISENCE KCA 0.03 99.8 60.5 101L Example 3 UNISENCE KCA 0.01 99.8 60.5 101L Example 4 PAS-880 0.05 99.8 55.0 Example 5 PAS-880 0.10 99.8 52.0 Example 6 PAS-J-81 0.10 99.8 57.0 Example 7 PAA-1123 0.10 99.8 57.0 Example 8 PAS-H-5L 0.05 99.6 55.0 Example 9 UNISENCE KCA 0.03 99.8 56.0 101L PAS-880 0.02 Comparative Polyvinyl alcohol 0.10 99.8 70.5 Example 1 Comparative Polyquaternium- 0.10 99.7 71.0 Example 2 10 Comparative Not applicable — 99.7 71.5 Example 3

As shown in Table 1, the separation membranes of Examples 1 to 9 and Comparative Examples 1 to 3 exhibited the same level of MgSO₄ rejection ratio.

The NaCl rejection ratio of the separation membranes of Examples 1 to 9 was lower than the NaCl rejection ratio of the separation membranes of Comparative Examples 1 to 3. The NaCl rejection ratio of the separation membranes of Example 1 to 9 was 60.5% at a maximum. The NaCl rejection ratio of the separation membranes of Comparative Examples 1 to 3 was 70.5% or more. For the separation membranes of Examples 1 to 9, the difference between the MgSO₄ rejection ratio and the NaCl rejection ratio was large. This means that the separation membranes of Examples 1 to 9 exhibited superior divalent ion selective-separation performance.

The separation membranes of Comparative Examples 1 and 2, despite having a surface coating, exhibited a NaCl rejection ratio similar to that of the separation membrane of Comparative Example 3 which had no surface coating. That is, the surface coatings of the separation membranes of Comparative Examples 1 and 2 made no contribution to improvement in divalent ion selective-separation performance.

The separation membrane of the present disclosure can be used as a RO reverse osmosis) membrane, a NF (nanofiltration) membrane, an UF (ultrafiltration) membrane, a MF (microfiltration) membrane, or a FO (forward osmosis) membrane.

The invention may be embodied in other forms without departing from the spirit or essential characteristics thereof. The embodiments disclosed in this specification are to be considered in all respects as illustrative and not limiting. The scope of the invention is indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are intended to be embraced therein. 

1. A separation membrane comprising: a separation functional layer composed of a polyamide containing, as a monomer unit, at least one selected from the group consisting of piperazine and a piperazine derivative; and a coating covering the separation functional layer and containing a polymer having a repeating unit represented by the following formula (1):

wherein N⁺ is a nitrogen atom constituting a quaternary ammonium cation, R¹ and R² are each independently a substituent containing a carbon atom bonded to the nitrogen atom, the coating is the uppermost layer of the separation membrane, and the separation membrane is a nanofiltration membrane.
 2. The separation membrane according to claim 1, wherein in the formula (1), R¹ and R² are each an alkyl group.
 3. The separation membrane according to claim 1, wherein in the formula (1), R1 and R2 are each a methyl group.
 4. The separation membrane according to claim 1, wherein in the formula (1), R1 is a methyl group, and R2 is a 3-chloro-2-hydroxypropyl group.
 5. The separation membrane according to claim 1, wherein in the formula (1), R1 is a methyl group, and R2 is a 2,3-epoxypropyl group.
 6. The separation membrane according to claim 1, wherein the polymer is a copolymer of a first monomer serving to form the repeating unit and a second monomer, and the second monomer has a reactive substituent capable of being chemically bonded to the separation functional layer.
 7. The separation membrane according to claim 6, wherein the reactive substituent is an epoxy group, a hydroxy group, an amino group, or an amide group.
 8. The separation membrane according to claim 6, wherein the second monomer is 3-chloro-2-hydroxypropyl diallylamine hydrochloride, allylamine, or acrylamide.
 9. The separation membrane according to claim 1, further comprising a support supporting the separation functional layer.
 10. The separation membrane according to claim 1, wherein the polymer has a reactive substituent capable of being chemically bonded to the separation functional layer.
 11. The separation membrane according to claim 1, wherein the polymer is a copolymer of a first monomer serving to form the repeating unit and a second monomer. 